An alternative technique for forcing ack parity is to take advantage of the fast retransmit algorithm contained in most modern TCP implementations [Ste94]. This algorithm is based on the premise that since TCP always acknowledges the last in-sequence packet it has received, a sender can infer a packet loss by observing duplicate acknowledgments. To make this algorithm efficient, the delayed acknowledgment mechanism is suspended when an out-of-sequence packet arrives. This rule leads to a simple mechanism for guaranteeing ack parity: during the data seeding phase we skip the first sequence number and thereby ensure that all data packets are sent, and received, out-of-sequence. Consequently, the receiver will immediately respond with an acknowledgment for each data packet received. The hole filling phase is then modified to transmit this first sequence number instead of the next in-sequence packet.